The present invention relates to a reversible thermochromic composition having an excellent discoloration sensitivity, which comprises a special electron accepting compound having ultraviolet absorbing capacity to exhibit an improved light-fastness, particularly during quenching, without impairing discoloration sensitivity and a product comprising same.
A thermochromic composition comprising an electron donating compound and an electron accepting compound in combination is known as disclosed in U.S. Pat. No. 3,560,229. Since the discoloration temperature of this composition is determined by the kind of the two compounds to be combined, it is very difficult to obtain a composition which can undergo discoloration at a desired temperature.
In order to solve this problem, the inventors proposed an invention which comprises using a specific alcohol, ester or ketone as a discoloration temperature adjustor to adjust the discoloration temperature to a desired value even when the kind of electron donating compound and electron accepting compound to be used in combination remain the same (U.S. Pat. No. 4,028,118, JP-B-6-59746 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent applicationxe2x80x9d)).
The inventors further invented a process which comprises using a special light-fastness providing agent and light stabilizer having electron acceptingness to improve the light-fastness of a reversible thermochromic composition in quenched state (U.S. Pat. No. 5,879,438).
The above proposed invention can attain free adjustment of discoloration temperature even when the kind of electron donating compound and electron accepting compound to be used in combination remain the same and exerts an excellent effect of remarkably improving light-fastness in quenched state. However, as the added amount of the light stabilizer increases, the discoloration sensitivity tends to fall. Therefore, the above cited invention is limited in its practical use.
The present invention provides a reversible thermochromic composition which can be freely adjusted in its discoloration temperature even if the kind of electron donating compound and electron accepting compound to be used in combination remain the same and exhibits a remarkably improved light-fastness during quenching.
The present invention lies in the following constitutions:
1. A reversible thermochromic composition having an improved light-fastness, which comprises
(a) an electron donating compound,
(b) an electron accepting compound having ultraviolet absorbing capacity which is represented by the following general formula I: 
xe2x80x83wherein X represents a hydrogen atom, xe2x80x94CnH2n+1 or xe2x80x94OCmH2m+1; m represents an integer of from 1 to 9; n represents an integer of from 1 to 10; and p represents an integer of from 1 to 3, and
(c) a discoloration temperature adjustor.
2. A reversible thermochromic composition having an improved light-fastness, which comprises
(a) an electron donating compound,
(b) an electron accepting compound having ultraviolet absorbing capacity which is represented by the following general formula I: 
xe2x80x83wherein X represents a hydrogen atom, xe2x80x94CnH2n+1 or xe2x80x94OCmH2m+1; m represents an integer of from 1 to 9; n represents an integer of from 1 to 10; and p represents an integer of from 1 to 3,
(c) a discoloration temperature adjustor, and
(d) an electron accepting light-fastness providing agent represented by the following general formula II: 
wherein n represents an integer of from 5 to 17 (straight-chain and branched); X represents a C1-C4 straight-chain or branched hydrocarbon or halogen; Y represents a C1-C4 straight-chain or branched hydrocarbon or halogen; p represents an integer of from 0 to 3; and m represents an integer of from 0 to 3.
3. A reversible thermochromic composition having an improved light-fastness, which comprises
(a) an electron donating compound,
(b) an electron accepting compound having ultraviolet absorbing capacity represented by the following general formula I: 
xe2x80x83wherein X represents a hydrogen atom, xe2x80x94CnH2n+1 or xe2x80x94OCmH2m+1; m represents an integer of from 1 to 9; n represents an integer of from 1 to 10; and p represents an integer of from 1 to 3,
(c) a discoloration temperature adjustor,
(d) an electron accepting light-fastness providing agent represented by the following general formula II: 
xe2x80x83wherein n represents an integer of from 5 to 17 (straight-chain and branched); X represents a C1-C4 straight-chain or branched hydrocarbon or halogen; Y represents a C1-C4 straight-chain or branched hydrocarbon or halogen; p represents an integer of from 0 to 3; and m represents an integer of from 0 to 3, and
(e) a light stabilizer.
4. The reversible thermochromic composition having an improved light-fastness according to any one of Clauses 1 to 3, which further comprises a microcapsule incorporating said reversible thermochromic composition therein.
5. The reversible thermochromic composition having an improved light-fastness according to any one of Clauses 1 to 4, which further comprises a color developer blended in said reversible thermochromic composition having an improved light-fastness.
6. The reversible thermochromic composition having an improved light-fastness according to Clause 5, wherein said color developer is a binder.
7. A molding resin composition comprising a synthetic resin and a reversible thermochromic composition according to any one of Clauses 1 to 4 blended therein.
8. A reversible thermochromic yarn comprising a reversible thermochromic composition according to any one of Clauses 1 to 4 and a thermoplastic resin.
9. A reversible thermochromic layer having an improved light-fastness according to any one of Clauses 1 to 8, which further comprises
(f) a layer of light stabilizer and/or metallic luster pigment provided on the surface of a layer formed by a reversible thermochromic composition.